One of the biggest problems caused by vehicles using fossil fuel, such as gasoline and diesel oil, is creation of air pollution. A technology of using a secondary battery, which can be charged and discharged, as a power source for vehicles has attracted considerable attention as one method of solving the above-mentioned problem. As a result, electric vehicles (EV), which are operated using only a battery, and hybrid electric vehicles (HEV), which jointly use a battery and a conventional engine, have been developed. Some of the electric vehicles and the hybrid electric vehicles are now being commercially used. A nickel-metal hydride (Ni-MH) secondary battery has been mainly used as the power source for the electric vehicles (EV) and the hybrid electric vehicles (HEV). In recent years, however, the use of a lithium-ion secondary battery has been attempted.
High output and large capacity are needed for such a secondary battery to be used as the power source for the electric vehicles (EV) and the hybrid electric vehicles (HEV). For this reason, a plurality of small-sized secondary batteries (unit cells) are connected in series or, according to circumstances, in series and parallel with each other so as to construct a battery module.
Generally, unit cells have low mechanical strength. For this reason, a battery module is manufactured by mounting a plurality of unit cells in a cartridge and electrically connecting a plurality of cartridges with each other. However, the cartridges increase the size of the battery module. In order to solve this problem, U.S. Patent Application Publication No. 2003-0017387 and U.S. Patent Application Publication No. 2003-0118898 disclose a medium- or large-sized battery module manufactured by horizontally stacking a plurality of unit cells and mounting pipe- or strip-shaped fixing members to upper and lower ends of the unit cells so as to maintain the stacking structure of the unit cells. However, these technologies have problems in that the unit cells and the fixing members are individually mounted to a housing member of the battery module, whereby the structure of the battery module is complicated and thus the assembly process of the battery module is complicated, and a flow channel of a cooling system is interrupted due to the fixing members, whereby the cooling efficiency of the battery module is reduced.
On the other hand, the battery module constructed in the horizontal stacking structure has a problem in that, when gas is generated in the unit cells during the operation of the battery module, the distance between electrodes of the unit cells is increased, and therefore, the battery efficiency is decreased. Furthermore, when the unit cells swell in the thickness direction of the unit cells due to the generated gas, the shape of the electrodes may be changed with the result that internal short circuits may occur.
Consequently, there is high necessity of a battery module constructed in a horizontal stacking structure that is capable of stably maintaining the stacking structure of unit cells, preventing the swelling of the unit cells, when gas is generated in the unit cells, by virtue of a structure in which the unit cells are in tight contact with each other, and maintaining the distance between electrodes of the unit cells, thereby preventing the lowering of the battery efficiency.